A tonicized chord is a chord to which a secondary dominant progresses.[2] For example, if chord ii comes after V/ii (read: five of two), then ii has been tonicized. So, in a piece in the key of C major, the key being tonicized, the key of ii, is D minor (D because D is the second scale degree in a C major scale, minor because to construct a triad over D using only the pitches available in the key of C major−i.e. no sharps, no flats−the triad must be minor--D, F A). The V/ii chord is composed of the pitches in a V chord in the key of ii (key of D minor). Those pitches include A, C sharp and E. In the key of C major, C sharp is an accidental. One can often find examples of tonicization by looking for accidentals as there are always accidentals involved in tonicization. However, it is important to note that the opposite is not true−just because there is an accidental does not mean that it is definitely a case of tonicization.

Only major and minor chords may be tonicized. Diminished and augmented chords cannot be tonicized because they do not represent key areas. For example, an A major chord (A, C sharp, E) may be tonicized because A major also represents a key area−the key of A major. However, an A diminished chord (A, C, E flat) may not be not be tonicized because "A diminished" could not be a key area; there is no key area in classical western music that has A, C and E flat−the pitches that make up the A diminished chord−as the first, third and fifth scale degrees, respectively. This holds true of all diminished and augmented chords.

Tonicizations may last for multiple chords. Taking the example given above with the chord progression V/ii → ii, it is possible to extend this sequence backwards. Instead of just V/ii → ii, there could be iv/ii → V/ii → ii (additionally, thinking about the last chord in the sequence: ii, as i/ii, it becomes clear why the phrase "temporary tonic"−see above−is often used in relation to tonicization). Though perceptions vary [3] as a general rule if a chord is treated as the tonic for longer than a phrase before returning to the previous key area, then the treatment is considered a modulation.[4]